Warning : Use the following information at your own
risk. While accuracy is one my goals, there is always the possibility that
some of the information could be wrong. There could be typos. I
could also be severely mistaken in some of my knowledge. This site is meant to help clarify certain concepts of ECG
and at no point should any life-or-death decision be made based upon the
information contained within. Remember, this is just some page on the
internet. (If you do find errors, please notify me by feedback.)

When using ECGs to determine what arrhythmia (if any) a patient has, it would
be great if we could simply look at it and immediately recognize the rhythm.
Perhaps you can do so successfully, but this is not to be encouraged.
Instead, you should use a system so that your interpretations of ECG after ECG
will be consistent.

The Paper

Figure 4-1 : The classic ECG rhythm

An ECG is printed on paper covered with a grid of
squares. Notice that five small squares on the paper form a larger
square.
The width of a single small square on ECG paper represents 0.04 seconds. To
successfully interpret ECGs, you must have this value committed to
memory. Do this now. If each small square represents
0.04 seconds, then a second will be 25 small squares across. If you
print out a minute's worth of your heart's electrical activity, the paper
would be 1500 small squares wide. If something on an ECG is, let's
say, 12 small squares in
width, that means that it lasted 12 x 0.04, or almost half a second. A
common length of an ECG printout is 6 seconds; this is known as a "six second
strip."

Figure 4-2 : A small square is
0.04 seconds

The width of one small square is 0.04 seconds (which is
equal to 40 milliseconds).

In figure 4-1, look for the baseline.
It is the line that would be perfectly straight and horizontal if not for those
vertical deflections. This is called the isoelectric line. On
a single ECG strip, the isoelectric line is usually about mid-height. The
exact vertical position on the paper can vary from ECG to ECG. When
looking at a rhythm, imagine the isoelectric line superimposed on top of the
rhythm.

The "height" of an ECG wave is called its amplitude. The
isoelectric line is considered to have an amplitude of zero. Anything
above the isoelectric line is positive; below the line is negative.

While the horizontal parts (i.e. the X-axis) of an ECG
measure time, the vertical parts (a.k.a. the amplitude) represent the "strength"
of the electricity at a given time. I put strength in quotations because
what it actually measures is "voltage" along a certain path. I won't go
into why I put voltage in quotations. Going back to the previous analogy,
note that the heart doesn't act as one big firecracker. Instead, it's like
a series of little firecrackers that go off at different times. At any
given time, the electricity around the beginning of the "fuse" could be going a
different direction from the electricity at the end of the fuse. (For
example, when the pacemaker first fires, it is unlikely that there is any major
electrical activity around the end of the path. When electricity finally
reaches and depolarizes the areas near the end, the areas that were depolarized
before it could already be in the process of repolarizing). The amplitude
(i.e. how high or low the ECG line is) at any given time represents the sum of
all of the little regions of electrical activity.

When the sum of all voltages is equal to zero, it remains on this line.
Hence the dramatic "flat-lining" that is so common in television shows.
Just because there are no deflections (and thus the ECG is a flat line) does not
necessarily mean the patient has no electrical activity in his heart. Know
that voltage can be negative. Think of negative voltage as pointing the
opposite direction as its positive equivalent. If all of the voltages in
the heart were cancelled out by an equal but opposite voltage, you might see a
net voltage of zero. Also, if all of the voltage is pointed
perpendicularly to the line of view, it will register as zero. Perhaps a
more likely reason for the flat line is that the treatment provider forgot to
hook the patient up to the machine. (Remember to always treat the patient
and not the machine.) When the net voltage is positive, the ECG deflects above this line. When the net
voltage is negative, it deflects below the line.

Look at the diagram below. (Note that I have superimposed a green line
over the isoelectric line.) What you see is the textbook, ideal Lead II
representation of the electrical activity in one heart beat. That means
that this person should see about seventy five of these patterns in one minute.
In the pattern, there are five letters : P,Q,R,S,T. They are in
alphabetical order so they are not that hard to remember. Each one of
these letters represents what is known as an ECG wave. Notice how
each one of them starts and stops on the isoelectric line. The term
complex is usually used to mean a group of adjacent waves.

The first little hump is known as
the P wave. It occurs when the atria depolarize (i.e. trigger). The
P wave (or its absence) will give you clues to where this electrical
activity started (ask yourself : did it start in the sinus node, the atria,
the ventricles, or somewhere in between?)

The next three waves constitute
the QRS complex. They represent the ventricles depolarizing.
These three are lumped together because a normal rhythm may not have all
three. Many times, you'll only see a R and an S. This is not
abnormal. If there are less than three, how do we know which one is
which? Well, the R wave is the first wave ABOVE the isoelectric
line. You then name the waves in relation to the R wave. If it
falls before the R wave, it is called the Q wave; after the R wave is
the S wave.

Figure 4-3 : The waves of an ECG (Note
: green line is superimposed over the
isoelectric line.)

If there are no upright waves in the QRS complex (and you are sure you aren't mistaking the T wave for part of the QRS), you
call the second upright wave R' (pronounced "R prime").

Everything that depolarizes must repolarize if it intends to be triggered
again. The repolarization of the atria usually occurs while the QRS is
occurring and is not really noticeable. The repolarization of the
ventricles, however, is represented by that large hump after the QRS complex.
This is known as the T wave.

Sometimes a U wave appears after the T wave. This is uncommon.

One of the first things you want to measure is the electrical rate of the
heart. I have used the term "electrical" because you CANNOT measure the
true heart rate with an ECG machine. The true heart rate is the rate at which
the heart pumps blood. This is detected by taking a pulse.
In the normal, healthy heart, the true rate will coincide with the electrical
rate. However, there are many reasons why there might not be a
corresponding pulse to go with an electrical beat.

The Caliper

An ECG caliper (sometimes plural like
scissors) is a tool that helps measure certain values. It
usually has no measuring ability of its own, but allows you to set its width
and then measure it against a more convenient portion of the ECG paper.
Although the tool is often marketed as an "ECG caliper", it is really a
general tool that can be used for many different things. In the
non-ECG world, I've heard them called "dividers."

Figure 4-4 : A typical ECG caliper

Step 1. Position the two arms of the caliper on the
each end of the width you intend to measure. Make sure that the arms
are perfectly horizontal (i.e. on the same vertical line).

Figure 4-5 : Positioning the arms of a
caliper on two adjacent QRS complexes.

Step 2. After you have positioned the caliper arms,
do not let them move (relative to each other). This would defeat the
purpose of using the calipers. Find a place on the paper that it will
be easy to see the lines. Keeping the two caliper arms on the same
vertical level, place the left arm of the caliper on the left side of a big
square. Count the number of small squares between the two arms.
(Remember that each big box is five small squares.) Multiply this by
0.04 to convert the number of small squares into seconds.

Figure 4-6 : Moving the caliper to a
more convenient place to count squares.

Of course, calipers are not absolutely necessary but they do make the job of
ECG interpretation much less tedious.

What is there to measure on an ECG? Well, for basic Lead II rhythm
interpretation, there are three things you want to be sure to measure every ECG.
They are : R-R interval, the PR interval, and the QRS width. We will learn
later what these values mean. For now, concentrate simply on being able to
measure them correctly.

The R-R interval

This is a measurement of the distance between two consecutive beats.
The R wave is usually chosen to do this because it is the tallest and most
conspicuous. In most rhythms (including normal rhythms), the R-to-R
interval will be the same as the P-to-P distance, or the distance between any
two analogous points on consecutive beats.

If the R-R interval is constant, no matter what two consecutive beats we
chose, we say the rhythm is regular. Do not confuse the terms
regular and normal. The term regular does not indicate
whether the rate is normal, fast, or slow, just that the beats are evenly
spaced. Irregular, therefore, means that waves are not evenly
spaced. Think of someone you know who has absolutely no rhythm.
Imagine this person trying to clap his or her hands to the beat of a song but
failing miserably. Some claps are too close together, some are too far
apart. This is what should come to mind when you hear the word
irregular in the context of ECG.

Rate is expressed in beats/min. It probably isn't accurate to say
"beats/min" in this case because you cannot tell if the heart is actually
pumping blood simply from an ECG. When using only the ECG, it is best to
refer to what you measure as the electrical rate and leave out the word
beats, saying simply "per minute" as in "The electrical rate is 75 per
minute."

How do you determine rate of the rhythm?

The more accurate method : 1500 / (# of small squares in R-R interval)

This should only be used if the rhythm is regular. If the R-R interval
fluctuates from beat to beat, then it should be obvious why you cannot use this
formula. (We use the number 1500 because there are 1500 small squares in a
minute.)

The quick estimation : count the number of electrical beats in a
six-second strip and multiply that number by 10.

This should be used if the
rhythm is not regular. It is not as accurate as method #1. (If you
use this method, every rate you get will be evenly divisible by 10. The
true heart rates tend not to be so "computationally friendly.")
Never assume that a strip is six seconds; check for tick marks or count the
large squares.

The PR interval

Often abbreviated PRI, the PR interval is the distance (time)
between the beginning of the P wave and the beginning of the QRS complex.
The R in PR interval refers to the R wave, but it is not always
measured to that wave. It actually ends at the beginning of whatever the
first wave in the QRS complex is. It might have been more accurately
called "P-to-QRS interval," but that would have been a mouthful.

A normal PRI should be
in the range of 0.12 - 0.20 seconds. Memorize this. Since we
already know that each square represents 0.04 sec, we are able to calculate that
0.12 seconds is three squares and that 0.20 seconds is five squares.

Figure 4-7 : The PRI

In a normal rhythm, every P wave will be followed by a QRS complex.
This is the same thing as saying that the ratio of P waves : QRS complexes is 1 : 1.
In some rhythms, however, you don't see this 1:1 ratio. The absence of
this 1:1 ratio provides a major clue as to what rhythm may be present. In
some cases, it will be impossible to measure the PRI.

The QRS width

Another important measurement is QRS width (also called QRS duration).
This is the measurement from the beginning of the first wave in the QRS to the end of the
last wave in the QRS. A normal QRS width should be less than
0.12 s. Some people say it should be less than 0.10 s, but both groups
agree that it should be less than 0.12 s. (In order to confuse the reader,
I have decided to use both values and frequently switch back and forth.) Below this value
(and thus of normal duration) it is called a
narrow QRS. When a QRS is longer than 0.10 - 0.12 seconds, it is
called a wide QRS.

The QRS begins at the
point where the PRI stops. This is either the beginning of the Q wave; if there is no Q wave,
it starts at the beginning of the R wave.

The end of the QRS can be tricky to find. The S wave (or whatever the last wave in the QRS is) ends
at the beginning of what is called the ST segment. This point
is sometimes called the J point. In the usual "textbook" ECG,
the S wave ends on the isoelectric line. (On figure 4-7, the QRS is easy to
spot.) However, in many cases, the ST segment is not so flat. In
Figure 4-8, notice that the ST segment starts below the isoelectric line.
The J point (i.e., the end of the QRS complex) appears like a bend in the
road.